As is well-known in the art, a wire-cut electric discharge machine has a wire electrode stretched between an upper guide and a lower guide and machines a workpiece by producing an electrical discharge between the wire electrode and the workpiece. The workpiece, secured to a table, is transported in X and Y directions along a machining contour in response to commands from a numerical control apparatus. When the wire electrode is tensioned normal to the table (workpiece), the upper and lower surfaces of the workpiece will be machined into contours which are identical. If the arrangement is such that the upper guide can be displaced in the X and Y directions (referred to as the U and V axes) to incline the wire electrode with respect to the workpiece as by displacing the upper guide in a direction at right angles to the direction of workpiece movement, then the upper and lower surfaces of the workpiece will not be cut to the same contour, and the surface cut by the wire electrode will be inclined. This is so-called taper cutting.
FIG. 1 is a view for explaining such taper cutting, in which a wire electrode WR is stretched between an upper guide UG and a lower guide DG at a predetermined angle of inclination with respect to a workpiece WK. If we take the lower plane BPL of the workpiece WK as the programmed plane (the upper plane APL of the workpiece WK may also serve as the programmed plane), and if we let a denote the taper angle, H the distance between the upper guide UG and lower guide DG, and h the distance from the lower guide DG to the lower plane BPL of the workpiece WK, then the offset d.sub.1 of the lower guide DG and the offset d.sub.2 of the upper guide UG with respect to the lower plane BPL of the workpiece, may be expressed as follows: ##EQU1## Note that d is the cut width.
Accordingly, if the movement of the upper guide UG between which the wire electrode WR is stretched is so controlled in relation to workpiece movement that the offsets d.sub.1, d.sub.2 remain constant, then taper cutting at the taper angle a can be carried out, as shown in FIG. 2. The dashed line and one-dot chain line in FIG. 2 indicate the paths of the upper and lower guides UG, DG, respectively.
In performing taper cutting with such a wire-cut electric discharge machine, a programmed path in the upper or lower plane of the workpiece, feedrate on the programmed path, taper angle a and distances H, h, etc., are commanded as set forth above and cutting is carried out in the manner commanded. According to this method, taper cutting can be carried out through simple control if the taper angle is constant.
With the foregoing method, however, it is difficult to perform cutting of the type in which the taper angle changes in continuous fashion. For example, cutting in accordance with the conventional method is difficult if the centers of circular arcs C.sub.u, C.sub.d (cut contours) on the upper and lower planes of a workpiece do not agree, as shown in FIG. 3, if the positions of rectangles R.sub.u, R.sub.d (cut contours) on the upper and lower planes of a workpiece are skewed with respect to each other, as shown in FIG. 4, or if the contours to be cut on the upper and lower planes of a workpiece differ.